Semitransparent PV cells go designer

Sarina Tracy, sarina.tracy@photonics.com

Solar panels are hard to miss. Whether you see a dozen perched on the roof of a house or a few thousand laid down in rows on a solar farm, their dark metallic hues and large presence are eye-catching. For the majority of their existence, solar panels have been limited to roofs and fields to ensure clear sun exposure. Recent technological developments, however, might force you to look elsewhere. Solar cells now can be transparent, colored and downright sneaky, poised for integration into any environment under the sun – literally.

“We think we can make solar panels more beautiful – any color a designer wants,” said Dr. Jay Guo, professor and leader of the Guo Research Group at the University of Michigan. “And we can vastly deploy these panels, even indoors.”

Guo and his team have engineered what are believed to be the first semitransparent, colored photovoltaic cells. Broadening the use of solar power while maintaining aesthetic appeal for all kinds of environments, this technology could become energy-harvesting billboards on the sides of buildings, solar window shades in our homes and even stained glass, Guo said.

“I think this offers a very different way of utilizing solar technology, rather than concentrating it in a small area,” he added. “Today, solar panels are black, and the only place you can put them on a building is the rooftop. And the rooftop of a typical high-rise is so tiny.”

Guo’s cells are designed to transmit or reflect certain colors, kicking energy from those wavelengths back to our eyes. In a cell patterned after an American flag and mounted in a palm-sized slide, for example, the red stripes and blue background consist of solar cells with semiconductor layers of amorphous silicon. The thickness of the layers dictates their color, and they are mechanically structured to transmit certain light wavelengths: Layers for the blue and red regions are 6 and 31 nm thick, respectively. Unlike previous cells, they do not rely upon dyes or microstructures that obscure whatever’s behind them.

The amorphous silicon layer is sandwiched between two semitransparent electrodes that absorb light and carry away electrical current. A hybrid structure of both organic and inorganic components allows the cells to hold their color, be 10 times thinner than previous structures, and exhibit 100 percent quantum efficiency. A 1-m² panel could generate enough electricity to power fluorescent lightbulbs and small electronic gadgets, Guo said. The cells’ hues also stay consistent at any viewing angle, allowing manufacturers to lock color for precise pictures or patterns while ensuring light retention at any sun position. Traditional solar panels have to pivot to follow sunlight.

“Solar energy is essentially inexhaustible, and it’s the only energy source that can sustain us long-term,” Guo said. “We have to figure out how to use as much of it as we can.”